The manufacture of semiconductor devices requires hundreds of processing steps. In turn, each process step may have several controllable parameters, or inputs, that effect the outcome of the process step, subsequent process steps, and/or the process as a whole. The typical semiconductor device fabrication process thus has a thousand or more controllable inputs that may impact process yield. Process models that attempt to include all process inputs and/or provide intelligent system models of each process are generally impractical for process control in terms of both computational time and expense. As a result, practical process control requires a process model that excludes process steps and inputs that do not have a significant impact on process yield.